Biological prostheses or “bioprostheses” are devices derived at least partially from processed biological tissues to be used for implantation into humans. Examples of bioprostheses that are currently used or in development include heart valves, vascular grafts, ligament substitutes, pericardial patches, and others. Even though much is now known about biological tissue, bioprostheses and the processing, assembly, and performance thereof, there are still deficiencies that need to be overcome to provide a bioprosthesis that preserves the native tissue properties while optimizing tissue biomechanics, minimizing calcification, and/or rendering the treated tissue hemocompatible.
For example, the biological tissue that is harvested from a donor must be stored under proper conditions, and in proper solutions, to preserve the native properties of the tissue prior to and during the tissue processing steps that are to be subsequently undertaken. In addition, the harvested biological tissue should be stored in a manner that mitigates or even reduces the bioburden of the harvested tissue.
Further, the primary component of biological tissues used to fabricate many bioprostheses is collagen, a term used here in a generic sense to refer to a family of related extracellular proteins. Collagen molecules assemble to form microfibrils, which in turn assemble into fibrils, resulting in collagen fibers. The amino acids that make up the collagen molecules contain side groups that represent sites for potential chemical reaction on these molecules. Because collagenous tissues degrade rapidly upon implantation into a host recipient, it is necessary to stabilize the tissue if it is to be used for long-term clinical applications. Chemical stabilization by cross-linking collagen molecules within the tissue (also known as tissue fixation) is well-known, and glutaraldehyde is commonly used to cross-link tissue.
Unfortunately, glutaraldehyde-fixed bioprosthetic tissues tend to become calcified over time. The mechanism by which calcification occurs in glutaraldehyde-fixed bioprosthetic tissue has not been fully explained, and many factors have been thought to influence the rate of calcification. In general, the calcification phenomenon has been characterized as being due to intrinsic causes (i.e., causes inherently contained within the tissue itself) and extrinsic causes (i.e., causes from outside the tissue itself, such as infection, patient's age, existing metabolic disorders, flow disturbances, etc.). One intrinsic cause of calcification has been shown to be the presence of phospholipids in the harvested tissues. See e.g., Cunanan et al., Tissue characterization and calcification potential of commercial bioprosthetic heart valves, Annals Thoracic Surgery, 2001; 71:S417-S421. Therefore, it is desirable to mitigate or inhibit the calcification of the tissue in order to increase the usable life of any bioprosthesis that is implanted into a human host.